To live, aerobic organisms metabolize oxygen to generate energy. During this process, cells create reactive oxygen species (ROS). ROS react with all cellular constituents, including lipids, proteins, and DNA. Such oxidative damage has been associated with the aging process and human disease, namely cancer and neurodegeneration. We have worked to define the biochemical and cellular processes for repairing oxidative DNA damage. In particular, we have delineated the structure-function mechanisms and biological contributions of specific proteins that participate in the base excision repair (BER) pathway. This process involves the recognition and excision of DNA damage, and restoration of the native genetic material. Defects in DNA repair give rise to mutations or cell death, leading to the development of disease.[unreadable] [unreadable] Much of our effort has involved defining the biochemical functions of Ape1, the major human protein for repairing abasic (AP) sites in DNA, a frequent genetic damage. We have demonstrated that Ape1 contributes to the repair of 3?-modifications in DNA as well, including mismatches, phosphate groups, phosphogycolates, and tyrosyl residues. Our more recent work has found that Ape1 cleaves at AP sites in single-stranded regions of complex, biologically-relevant DNA structures, such as bubble and fork intermediates. These findings expand the known repertoire of substrates processed by this enzyme, and suggest novel functions for Ape1 likely coupled to transcription and/or replication. Our recent work has focused on potential mechanisms of regulating Ape1 repair activities. For instance, we have demonstrated that the single-stranded DNA binding protein RPA inhibits promiscuous AP site incision by Ape1. In addition, CSB, a transcription-related repair protein defective in the human premature aging disorder Cockayne Syndrome, was found to activate Ape1 cleavage at AP sites in transcription bubble intermediates. Finally, our studies have discovered that the environmental metal, lead, is a potent inhibitor of Ape1 activity, suggesting a means by which this heavy metal may elicit its co-carcinogenic effects. We are currently designing methods to strategically regulate Ape1 repair activity in cells in the hopes of developing more effective anti-cancer treatment paradigms.[unreadable] [unreadable] In addition to the investigations above, we have initiated studies to determine the biochemical and cellular contributions of XRCC1, a major single-strand break repair (SSBR) factor. This protein functions primarily as a scaffold component, orchestrating specific protein-protein interactions required for efficient DNA repair. Recent work has identified associations of XRCC1 with proteins defective in human neurodegenerative disorders AOA1 (Aprataxin) and SCAN1 (TDP1). Our studies suggest a link of XRCC1 to replication via an interaction with PCNA, argue against a role for XRCC1 in the early steps of BER, and indicate a biologically-relevant role for its interaction with DNA polymerase beta and in the subsequent steps of SSBR, specifically DNA nick ligation. Ongoing studies using animal models (and derived cells) are determining the relationship of XRCC1 and oxidative DNA damage repair to aging and age-related disease, namely neurodegeneration. Additionally, we are determining the contribution, if any, of human XRCC1 and associated protein variants to impaired cellular responses that are related to disease manifestation.